Staged isenthalpic-isentropic expansion of gas from a pressurized liquefied state to a terminal storage state



Sheet o f 5 March 18, 1969 J. s. swf-:ARINGEN STAOEDISENTHAERIc-ISENTROPIO EXPANSION OR GAS EROM A PRESSURIZED LIQUEFIEDSTATE TO A TERMINAL STORAGE STATE Filed Nov. 7. 1966 l N VEN TOR.

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ATTO/P/VEVJ March 18, 969 .1, s. SWEARINGEN 3,433,026

STAGED ISENTHALPICISENTROPIC EXPANSION OF GAS FROM A PRESSURIZEDLIQUEFIED STATE TO A TERMINAL STORAGE STATE ATR/VE VJ FROM March 18,1969 1. s. SWEARINGEN STAGED ISENTHALPIC-ISENTROPIC EXPANSION OF GAS APRESSURIZED LIQUEFIED STATE TO A TERMINAL STORAGE STATE Sheet Filed Nov.7. 1966 www Jaa/502? 5. Swear/27992? INVENTOR.

5M ,4 Trae/wxs United States Patent O STAGED ISENTHALPIC-ISENTROPICEXPANSION OF GAS FROM A PRESSURIZED LIQUEFIED STATE TO A TERMINALSTORAGE STATE Judson S. Swearingen, 500 Bel Air Road, Los Angeles,Calif. 90024 Filed Nov. 7, 1966, Ser. No. 592,563

U.S. Cl. 62-23 Int. Cl. F25j 3/06 7 Claims ABSTRACT F THE DISCLOSUREThis invention relates to a method of more efliciently liquefying gases.More particularly, it relates to a novel method of taking energy out ofa cold stream of pressurized liquefied natural gas during thedepressurization of the liquefied natural gas to atmospheric pressure.

In most of the processes used today to liquefy natural gas, the naturalgas to be liquefied is first compressed and liquefied at a pressureconsiderably above that at which it is to be stored or used. Thispressurized liquid (usually at its boiling point) then has its pressurereleased through an expansion orifice or valve commonly known as aJoule-Thomson nozzle to atmospheric pressure for storage in a cryostat.In such process, a portion of depressurized liquid vaporizes. This vaporis disengaged from the fluid and recycled or otherwise disposed of. Theresidual liquid constitutes the storable liquid derived from theprocess.

If, instead of isenthalpically expanding the pressurized liquefiednatural gas through a Joule-Thomson nozzle to atmospheric pressure, theliquefied natural gas were expanded in an expansion engine so itsexpansion could take place isentropically, its energy of expansion couldbe removed as power and a heat equivalent to this power would be removedfrom the system. Various types of liquid turbines and piston expansionengines have been considered for this expansion. However, the presenceof a comparatively dense liquid along with a light vapor makes turbineexpansion of a saturated pressurized liquefied gas diflicult and at lowefficiency. Expansion of a saturated pressurized liqueed gas in apiston-type expansion engine is diflicult because of the effect that theliquid has on the pistons, seals and valves. Accordingly, it iscustomary to flash the pressurized liquefied natural gas directly toatmospheric pressure through a Joule-Thomson nozzle, losing theexpansion energy which could be conserved if the pressurized liquid wasexpanded isentropically.

I have found that, if, instead of flashing the pressurized liquefiednatural gas isenthalpically to its terminal pressure, it is flashed toan intermediate pressure, the flash gas produced by the initialexpansion can be separated from the remaining liquid, expandedisentropically and a large amount of the expansion energy may berecovered, plus an increase in liquid end product will be realized.

The intermediate flashing of the pressurized liquid is particularlyuseful in the liquefaction of methane, and a 3,433,026 Patented Mar. 18,1969 description of the liquefaction of methane will be used as anillustrative embodiment of the invention.

It is an object of this invention to increase the amount of liquefiedgas that is retained when the pressure of the liquefied gas is reducedfrom process pressure to terminal pressure for storage.

It is another object of this invention to obtain useful work from thereduction of the pressure of liquefied natural gas, produced by anatural gas liquefication process, to a storage or terminal pressure.

It is an object of the present invention to provide in the process ofliquefying natural gas the flashing of the pressurized liquefied naturalgas to an intermediate pressure stage and then isentropically expandingthe vapor, resulting from the flashing, to the terminal pressure stageto provide useful work and additional liquefied end product.

It is another object to provide in the liquefaction process of naturalgas the flashing of the pressurized liquefied natural gas to a pointabove its terminal pressure, d isengaging the vapor resulting from theflashing, and expanding such vapor isentropically in a turboexpander toits terminal pressure.

It is a further object of this invention to decrease the amount ofliquefied gas that vaporizes when the pressure of the liquefied gas isreduced to a terminal pressure for storage by expanding the pressurizedliquefied gas isenthalpically to an intermediate pressure, disengagingthe flash vapor therefrom, isentropically expanding the intermediatelydisengaged vapor to a terminal pressure, and isenthalpically expandingthe intermediately disengaged liquid to the terminal pressure.

It is still a further object in the liquefaction of a gas from apressurized liquid state to a terminal storable state to isenthalpicallyexpand the pressurized liquid in intermediate stages and, after eachisenthalpic expansion, isentropically expand the resulting vapor to thenext subsequent stage.

Other objects and advantages of this invention will become apparent fromthe following description taken in connection with the accompanyingdrawing, wherein is set forth an illustrative embodiment of theinvention.

FIG. l is a flow diagram for the liquefaction of methane usingcompression, turboexpansion and mechanical refrigeration;

FIG. 2 is a pressure volume diagram, in which area represents work orexpansion energy;

FIG. 3 is a flow diagram of the two stage depressurization taught by thepresent invention; and

FIG. 4 is a flow diagram of FIG. l modified to include the two stagedepressurization of FIG. 3.

In general, liquefaction of natural gas is carried out either bycascaded mechanical refrigeration or by the use of turboexpanders or acombination of the two. FIG. 1 illustrates a flow diagram for methaneliquefaction using compression, turboexpansion and mechanicalrefrigeration.

As can be seen, methane, at 200 p.s.i.a. and 105 F. is fed into thesystem. The feed stream is joined by a recycle stream of the samepressure from compressor 10 and an additional recycle stream of 200p.s.i.a, from a heat exchanger 18. The combined stream is compressed to1000 p.s.i.a. in stages by compressor 12 operated from expander 14 andmain compressor 16. The compressed stream is then fed into the heatexchanger or mechanical refrigeration device 18 where it is cooled fromthe 105 F, temperature at which it enters. The larger portion of thestream is taken from the heat exchanger when it reaches minus F. andpassed through the turboexpander 14.

The 1000 p.s.i.a., minus 70 F. stream is expanded to 200 p.s.i.a. andfed into a separator 20 where the liquid is drained olf. The 200p.s.i.a. vapor is recirculated back through the heat exchanger 18. Theenergy derived from the expander 14 by the expansion is used to drivethe compressor 12.

The remaining small portion of the 1000 lb. stream is further cooled inthe heat exchanger 18 to minus 178 F., at which point lit becomesliquid. The pressurized liquid stream is released through a throttlingvalve 22 to 200 p.s.i.a. This liquid is combined with the liquidrecovered from the separator 20 and the combined 200 p.s.i.a. streamflashed to atmospheric pressure through a Joule- Thomson nozzle '26 intoa cryostat 27.

The ash gas resulting from the Joule-Thomson reaction is recirculatedback through the heat exchanger 18, recovering its refrigeration andrecompressed by the co-mpressor for recycling. The uncondensed portionof the 200 p.s.i.a. turboexpander discharge stream is also circulatedback through the heat exchanger 18, recovering its refrigeration andalso recycled. The power generated by the expansion to the 200 p.s.i.a.level in expander portion 14 of the turboexpander is recovered by thecompressor portion 12 operated in series with the main high-pressurecompressor 16.

This system has been designed to obtain eicient use of compression,turboexpansion and mechanical refrigeration in the liquefaction ofmethane. However, as a result of flashing the 200 p.s.i.a. liquid streamto atmospheric pressure through the Joule-Thomson nozzle 26, there is aloss of energy. A more complete description of the ef- `ciency of thissystem is contained in my paper which was published in the August 1966,issue of Hydrocarbon Processing.

Although it has been realized that it would be more desirable toisentropically expand the 200 p.s.i.a. stream to atmospheric pressure,whereby its energy of expansion can be removed as power and the heatequivalent of this power removed from the system, thereby condensingmore liquid, no satisfactory solution has been developed. Althoughpiston engines and liquid turbines have been considered, the presence ofthe comparatively dense liquid along with the light vapor makes theexpansion difficult and, accordingly, has not been used.

However, it has been found that if the 200 p.s.i.a. cold stream ofliquid methane is flashed only part Way to its terminal pressure, suchas to 80 p.s.i.a., instead of all the way to atmospheric pressure, thenthe amount of flash gas at the intermediate pressure is approximately inthe order of one-half of total ilash gas that would be produced at theterminal pressure. This intermediate stage flash gas may be disengagedand made available at the intermediate pressure for isentropicexpansion. The isentropic expansion of this intermediate pressure gascan be carried out in a suitable turboexpander to recover a large amountof expansion energy. Expansion of this intermediate pressure ash gasthrough expansion ratios frequently encountered in such processes, suchas 3 to 1 or 6 to 1, produces approximately 8.5% liquid from the exhaustof the turboexpander and are reasonable operating con ditions for aturboexpander.

FIG. Z is a pressure Volume diagram in which area represents work orexpansion energy and illustrates the amount of expansion energyrecoverable by the isentropical expansion of 80 p.s.i.a. flash gas. Thediagram ABCO represents the total expansion energy removable by 100%eicient isentropic expansion. Simple flashing or isenthalpic expansionfollows the dashed line BB'C. Separation of the liquid at B and theisentropic expansion of the residual intermediate pressure gas to thedischarge pressure of 14.7 p.s.i.a. follows the solid line DD, and thearea representing expansion energy is shown by the diagram ADDO, whichin this illustration is approximately 55% of the area of ABCO'. 80% ormore of this 55% available energy may be recovered by the turboexpander,i

Accordingly, FIG. 3 illustrates the equipment arrangement for practicingmy invention and FIG. 4 illustrates the addition of my invention to amethane liquefaction process using compression, turboexpansion andmechanical refrigeration, such as is illustrated by the flow diagram ofFIG. l. The elements of FIG. 4 that are common to either FIG. l or FIG.3 are designated by the same number in both figures.

Referring now to FIGS. 3 and 4, instead of flashing the 200 p.s.i.a.combined liquid stream through the Joule- Thomson nozzle 26, as shown inFIG. l, the 200 p.s.i.a. stream is released to an intermediate pressureof p.s.i.a. by a valve 28. The resultant stream is fed into separator 30which disengages the 80 p.s.i.a. vapor from the liquid.

The 80 p.s.i.a. liquid recovered in the separator is flashed through aJoule-Thomson valve 32 to the atmosphere for delivery to cryostat 34 forstorage. The disengaged 80 p.s.i.a. vapor is fed into a turboexpander 36Where it is isentropically expanded to atmospheric pressure. The exhaustfrom the turboexpander 36 which is now at 14.7 p.s.i.a. is combined withthe 14.7 p.s.i.a. stream resulting from the isenthalpic expansion of the8() p.s.i.a. liquid by the Joule-Thomson nozzle 32. The combined streamis fed into cryostat 34, which will store the liquid product and permitthe disengagement of the super cooled 14.7 p.s.i.a. vapor which isrecirculated in the system in the same manner as the vapor from cryostat27 of FIG. 1.

In the process depicted by FIG. 1, if the pressurized liquid methane atits boiling point of 200 p.s.i.a. is released directly to atmosphericpressure (14.7 p.s.i.a.) through an orifice or expansion valve 67.8% ofthe stream would remain unvaporized.

It has been determined that if the 200 p.s.i.a. liquid stream wereexpanded to an intermediate pressure of 80 p.s.i.a. through a valve andthe vapor disengaged and the disengaged liquid expanded through anothervalve to 14.7 p.s.i.a., the disengagement of the vapor from thelast-mentioned expansion step would leave 68.5% liquid from the initial200 p.s.i.a. liquid. Expansion through a high-etliciency turboexpanderof the flash gas disengaged at the intermediate pressure will condense aportion of it equivalent to 2.6% of the initial liquid. These twoquantities of expanded gas amount to 71.1% of the original pressurizedliquid, thus resulting in a gain of 3.3% in product liquid. This is anactual increase in product yield of 3.3/67.8, which is equal to 4.88%.

Not only is more liquid product recoverble, but the energy developed bythe expansion of the 80 p.s.i.a. vapor stream in the turboexpander canbe utilized to compress some of the gas at some stage in the process,thereby further conserving energy.

Instead of having just one intermediate stage, ash gas may be disengagedat two or more successively lower intermediate pressures. In such case,each flash gas stream may be expanded, either in a separate expander orin an expander having intermediate gas introduction pressure points.Such a method would recover more power than the expansion of a singleflash gas stream.

It has also been found that if the natural gas being expanded is not apure substance nor a constant boiling mixture, that there are otheradvantages to the staged llashing. Flashing of such a liquid mixture toan intermediate pressure instead of t0 the final terminal pressureconserves liquid in such instances. Then the expansion of theintermediate flash gas in a t-urboexpander refrigerates it and condensesliquid from it. The liquid thus produced is richer in the lower-boilingconstituents and thus increases the recovery of this liqiud.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

It will be understood that certain features and succombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed 1. In theliquefaction process of natural gas to derive a liquid storable at aterminal pressure and wherein the natural gas is initially liquefied ata pressure substantially above said terminal pressure, the improvementwhich comprises: isenthalpically said pressurized liquid natural gas toan intermediate pressure below said elevated pressure but above saidterminal pressure; disengaging the vapor resulting from the isenthalpicexpansion to the intermediate pressure; isentropically expanding thedisengaged vapor to said terminal pressure to further cool the vapor andcause a portion of the vapor to condense, and isenthalpically expandingthe remaining liquid natural gas from the intermediate pressure to saidterminal pressure.

2. The process set forth in claim 1 in which the terminal pressure isapproximately atmospheric pressure.

3. The process set forth in claim 2 characterized in that theisenthalpic expansion of the liquid natural gas and the isentropicexpansion of the vapor produced thereby is in multiple stages from theprocess pressure level to the terminal storage pressure level.

4. In a process for the liquefaction of methane to derive liquid methanestorable at approximately atmospheric pressure, the improvementcomprising compressing the methane to a predetermined elevated pressure,cooling said pressurized stream to the neighborhood of minus 70 F.,isentropically expanding a large portion of the pressurized stream toapproximately 200 p.s.i.a. and disengaging the liquid formed from saidexpansion; cooling and liquefying the remaining small portion of thestream at minus 178 F. after which it is isenthalpically expanded toapproximately 200 p.s.i.a., combining the liquid derived from theisentropic expansion with liquid from the isenthalpic expansion,isenthalpically expanding the composite 200 p.s.i.a. stream to anintermediate pressure; disengaging ash vapor formed from saidisenthalpic expansion at said intermediate pressure; isentropicallyexpanding said intermediately disengaged vapor to the terminal pressure,and isenthalpically expanding said intermediately disengaged liquid tosaid terminal pressure, combining both expanded terminal pressurestreams and disengaging the vapor from said combined terminal pressurestreams.

5. The process set forth in claim 4 characterized in that theintermediate pressure to which the 200 p.s.i.a.

stream is isenthalpically expanded is approximately 8O p.s.1.a.

6. The process set forth in claim 4 characterized in that the 200p.s.i.a. stream is isenthalpically expanded at several stages between200 p.s.i.a. and the terminal storage pressure and that after eachisenthalpic expansion the vapor is disengaged and isentropicallyexpanded to the succeeding lower isenthalpic expansion pressure.

7. A process for the liquefaction of natural gas, comprising compressingthe natural gas to a predetermined elevated pressure, cooling saidpressurized gas in a heat exchanger, isentropically expanding a largeportion of the pressurized stream to a lower pressure that is well aboveatmospheric pressure, disengaging the liquid formed from said expansion;cooling and liquefying the remaining small portion of the natural gasstream after which it is isenthalpically expanded to the same pressurethat the larger portion was expanded to isentropically, cornbining theliquid derived from the isentropic expansion with liquid from theisenthalpic expansion, isenthalpically expanding the composite stream toan intermediate pressure between the lower pressure to which the twoportions were initially expanded and atmospheric pressure; disengagingHash vapor formed from said isenthalpic expansion at said intermediatepressure; isentropically expanding said intermediately disengaged vaporto a terminal pressure, and isenthalpically expanding saidintermediately disengaged liquid to said terminal pressure, combiningboth expanded terminal pressure streams of isentropically expanded vaporand last mentioned isenthalpically expanded liquid and placing liquidfrom said isentropic and isenthalpic expansions to terminal pressure instorage at said terminal pressure.

References Cited UNITED STATES PATENTS 2,677,945 5 1954 Miller.3,160,489 12/1964 Broco 62-23 XR 3,360,944 1/1968 Knapp 62-23 XR2,265,558 12/1941 Ward 62-39 XR 2,583,090 l/1952 Cost 62-39 XR 2,900,7978/ 1959 Kurata et al. 2,901,326 8/1959 Kurata et al. 2,903,858 9/ 1959Bocquet. 2,952,984 9/ 1960 Marshall 62-27 3,203,191 8/1965 French 62-93,236,057 2/1966 Tafreshi 62-23 XR 3,292,381 12/1966 Bludworth 62-27 XRNORMAN YUDKOFF, Primary Examz'nler.

V. W. PRETKA, Assistant Examiner.

U.S. Cl. X.R.

